Multi-layer film heaters in strip form



MULTI-LAYER FILM HEATERS IN STRIP FORM Filed Feb. 8, 1960 26 'lll 3 3INVENTOR. 27 I BOHDAN HURKO BY Z of 3 )6 His ATTORNEY Elite 3 957315Patented Dec. 4, 1962.

3&57515 MUi'lll-LAYER FELM HEATERS llN FURM Bohdan Hurlro, Louisville,Ky, assignor to General Else" tric Qompany, a corporation of New YorkFiled Feb. 8, 1966, fier. No. 7,181 7 tilahns. (til. 219-67) The presentinvention relates to resistance heating units and particularly to filmheaters which would be suitable for use as either the surface heatingunit for an electric range or for an oven heating unit where the unithas to operate at a temperature of about 1500" P. and with a poweroutput of nearly forty watts per square inch.

Present metallic sheath type resistance heaters which are used as boththe surface and oven heating units for electric ranges have more thanforty years of research and development work behind them. Since theyhave been improved to such a degree and used successfully for so manyyears, it is believed possible to state at the present time that thedesign of the sheath type heating unit is close to perfection. Otherheat sources have been investigated such as induction heating but theone that appears most promising as an improvement in this art is a filmheater using multiple film layers of noble metal such as gold andplatinum.

The most common type of film heater uses a conducting film of a thinlayer of tin oxide. A metal base is first covered with an insulatinglayer and then the tin oxide film is supplied. Relatively low operatingtemperature, low watts density and high resistance makes this type offilm heater unsuitable for surface heating unit application.

A principal object of the present invention is to provide a film heaterthat has heating characteristics that are comparable to a metallicsheath type resistance heater.

A further object of the present invention is to provide a novel surfaceheating unit of the film heater type which is easy to clean and presentsan attractive appearance.

A further object of the present invention is to design a resistanceheating unit which has a low thermal mass so that the unit will heat upand cool down at faster rates than the standard sheath type resistanceheater.

A further object of the present invention is to design a novelresistance heating unit of the film heater type that has a more uniformtemperature distribution and more heated surface for transmitting theelectrical energy in the form of heat to the utensil in which the foodis to be cooked.

A still further object of the present invention is to provide a filmheater in strip form of multiple layers of noble metal to prevent thecrystallization of the conductive layers at high temperatures.

A preferred embodiment of the present invention incorporates a basematerial with a high resistivity and dielectric strength such as Vycorwhich is a trademark of the Corning Glass Works and consists ofapproximately 96% silica. Bonded to one side of the base member is acontinuous strip of metallic film which is built up of multi-layers thatinclude at least one inner layer of platinum and an outer layer of gold.The layer of gold is the conductive layer and it has a low enoughresistance to provide the necessary power density. The gold film wouldbe satisfactory when used alone except that it begins to crystallize ata temperature of about 400 C. As the temperature increases the goldgrains grow and they gather metal into small globules that are connectedby a thin layer of the original finer crystalline structure layer. Inthe process of this local metal agglomeration the layer connecting thecoarse grains becomes thinner. Eventually fine cracks in the film areformed and the base material is exposed. When the film is heated by anelectric current, arcs are struck in these fine film cracks and themetal melts until the film breaks completely rendering the heaterunusable. It has been found that a material with a higher melting pointand a higher recrystallization temperature than the gold such asplatinum may be used with the gold to protect the conductive film fromthe complete agglomeration at the higher operating temperatures.

My invention will be better understood from the following descriptiontaken in connection with the accompanying drawing and its scope will bepointed out in the appended claims.

ZEEGURE l is a plan view of a surface heating unit embodying the presentinvention showing the film heater in strip form affixed to the undersideof a glass or ceramic base plate that is transparent to the infraredradiation.

FIGURE 2 is a cross-sectional elevational view of a surface heating unitembodying the invention taken through the center of the heating unit ofFIG. 1.

FIGURE 3 is a fragmentary plan view in cross-section taken on the lines3-3 of FIGURE 2 showing a method of pivotally mounting the surface unitto the cooktop of a range.

FIGURE 4 is a fragmentary elevational view in crosssection taken alongthe lines 4-4 o-f FIGURE 1 showing the multiple layers of noble metalforming a film heater in strip form on the underside of the base plate.

Turning now to a consideration of the drawing and in particular toFIGURE 1, there is shown a surface heating unit 161' that is acontinuous sheet of glass or ceramic material 11 such as Vycor. On theunderside of the base plate llll is formed a continuous film heater 12in strip form which is arranged in a serpentine shape. Each end of thestrip is provided with a terminal or electrode such as 13 and 14, whilethe mid-portion has a third terminal 15 so that different degrees ofheat can be selected as with standard surface heating units.

A better understanding of the construction can be had by a study of thecross-sectional view of FEGURE 2. The base plate 11 is confined within aflanged trim ring 16 by a bottom reflector pan 17 that is fastened tothe ring by rivet means such as 18. The surface unit is hinged to thecooktop 19 of a range by means of a hinge strap 29. A metal plate 21, isfastened by screws 22 to the lower extremity of trim ring 16 as is bestseen in FIGURE 3. The mid-portion of the hinge strap 20 is of L shape.The end of the strap that is fastened to the metal plate 21 ishorizontally disposed at right angles to the strap as at 23 forfastening the strap to the plate. At the opposite end of the strap thereis a vertical flattened portion 25 which includes an aperture forreceiving a hinge pin 26. This hinge pin extends through a similaraperture in a bracket member 27 that is fastened to a down-turned flange28 of the cooktop as is best seen in FIGURE 2. First the bracket 27 isassembled to the hinge strap 20, then the strap is assembled by means ofthe plate 21 of the surface unit. Accordingly, the surface unit isfastened to the cooktop by pivoting the unit upwardly and placing thebracket 27 in the location as shown in FIG- URE 2 so that a screw 29 maybe threaded through the bracket 27 and flange 28.

The metal plate 21 also supports a terminal block 30 which is of moldedinsulating material such as a phenolic resin. It is of generally U-shapein plan view as is best seen in FIGURE 3. Elongated screws 31 extendthrough the arms of the U member for threading into the supporting plate21. A fiat strip of insulating sheet 32 is sandwiched between the plate21 and terminal member 3b as is seen in FIGURES 2 and 3 so that theterminal block may receive a down-turned tongue portion 33 of the glassbase plate 11 of the heating unit. The terminal block is equipped withthree spring blades 34 for making electrical contact with the terminalsof the film heater 12.

The free end of each spring blade has a contacting surface 35 of silveror the like for making good electrical connection with the film heater.Each spring blade 34 is provided with a terminal screw 36 so that a leadWire such as 37 may be connected thereto. Suitable openings (not shown)are made in the insulating sheet 32 so that a screw driver may extendtherethrough for fastening the lead wire 37 to terminal screw 36.

FIGURE 4 shows a cross-sectional elevational view of the surface unitwith the base plate ill and a film heater 12 on its underside. The filmheater is constructed with a first layer of platinum 4t} and anoverlying layer of gold 41. Because these metal films are very thin thecost of this type of heater is quite low. For example the material costof the film for a six inch diameter plate heater would be in theneighborhood of ten cents. To obtain even heat distribution it is bestto use the film in the form of relatively narrow bands or strips of fromone-fourth of an inch to one-half of an inch wide rather than to coverthe surface completely with the film. Also the use of the stripssimplifies the problem of making connection with the terminals of theheater. The width of the strip and the diameter of the surface unit offrom six to eight inches limits the choice of precious metal films tothose with a resistance in the range of 0.5 to 1.5 ohms per square. Agold film is the only one that has such a low resistance but it tends tobreak at higher temperatures and at a power density of approximately 30to 35 watts per square inch. The failure of thin gold coatings whenheated by an electric current to a temperature of between 1080 F. and1208" F. may be explained by examining the crystaline structure or" thecoating. Microscopic examination shows that at about 400 C. the goldfilm starts to crystallize. At higher temperatures the gold formsrelatively large grains and very thin passages. Fine cracks areeventually formed and the base material is exposed until arcs aregenerated and the metal starts to melt. Small droplets of melted goldpull together because of a very high surface tension and finally thefilm breaks. One solution to this problem is to select a noble metalwith a higher melting point so that recrystallization will occur at amuch higher temperature. Platinum is such a metal since its meltingpoint is 3224 F. or 1279 F. higher than the melting point of gold.However, platinum films have a resistance in the range of 13 to 39 ohmsper square which is too high for a surface unit.

This invention includes the use of multiple layers of gold and platinumfilms or similar low and high resistance combinations in order to obtainthe same result. The total resistance of a double film is approximatelyequal to the lower resistance of the two components. For example, usingplatinum and gold film the total resistance is that of the resistance ofthe gold film because both layers are connected in parallel. Theplatinum with a much higher melting point retards crystallization andmigration of the gold. The multiple layers are more stable at highertemperatures and they allow a much higher power density, that is, morewatts per square inch.

It was discovered that the gold and platinum films change their highluster to a frosty finish when heated to a high temperature. Microscopicexamination shows that the films consists of small metal grains whichstart to grow at higher temperatures. Coarse grain diffuse the reflectedlight and the film loses its high mirror finish. When the grains grow insize they pull the metal together and the space between them becomesextremely thin. Films with a small addition of rhodium have a much finercrystalline structure. Also the grain growth is much lower at highertemperature. This is because the molecules of rhodium act as graingrowth nuclei and promote fine crystal characteristics of the films.

A detail that is incorporated with this invention is a pilot light thatis positioned under the center of the surface unit and is capable ofdirecting light through a tube 46 that is vertically positioned in thecenter of the reflected pan 17 for lighting up the center of the baseplate 11. This pilot light is fastened onto a supporting bracket 4-7that is in turn assembled with the terminal block 3t} by means of thefastening screws 31 as is best seen in FIG- URES 2 and 3.

During the life tests of this type of film heater failure has occurredmore often in the terminals where small sparks develop, especially whenthe circuit is closed by a switch. The conducting films are extremelythin and ven small arcing causes tiny breaks in the film and overheatingof tie terminals which eventually progress and finally open the circuit.The design of the present invention allows the flm heaters to operate ata much higher temperature without a failure. The base plate llll hasnwardly extending tongue at one side thereof so er by means of thereflector pan 17 as E 2. The base plate 111 which is of is a very poorthermal conductor and n that there is a temperature drop of platinumfilms are obtained in liquid form are es. entia ly solutions in volatileoils of organic .d and pl ".im compounds. When the films are 1 baseplate and heated to a curing tem- -.ture the organic parts are burnedout and the metallic film is left on the surface. These liquids containsome base metal organic compounds such as Bi, Sn, and Cr which oxidizeand serve as fluxes to fix the metal firmly to the supporting material.The pattern of the film heater is printed on a silk screen or stainlesssteel screen and these screens are covered with photo emulsion which isapplied thinly so it does not add to the thickness of the screen, as itfills only the voids in the mesh. When the pattern is printed on thescreen the emulsion is removed so that the liquid can be passed. Arubber squeegee with a V-sharpened edge forces the liquid materialthrough the screen and onto the base plate. Then the films are driedeither at room temperature for one hour or they may be dried faster bypassing them through a warming chamber. After the films are dried, theprinted plates are placed on moving belts in a furnece. During thisfiring process it is very important to allow all organic material toburn out. The furnace atmosphere should be strongly oxidizing. Thisorganic matter is especially dificult to burn out completely when thefilms are thick. Any small residue of these materials left in the filmsmake the films much weaker. After the firing process of approximately 60minutes the base plate is cooled down in the furnace and the nextcoating is applied.

There are many advantages in a surface heating unit made with filmheaters in strip form. The most important advantage is its clean andattractive appearance. This type of heater has a low thermal mass,therefore it stores less heat when heated up and it heats up and coolsdown faster than a sheath type heating unit. it has a more eventemperature distribution, and has more heated surface in contact withthe cooking utensil. it has been found that it performs better at lowertemperatures than a sheath type surface unit. Finally, it is simple tomanufacture and its cost is low.

Modifications of this invention will occur to those skilled in this artand it is to be understood, therefore, that this invention is notlimited to the particular embodiment disclosed but that it is intendedto cover all modifications which are within the true spirit and scope ofthis invention as claimed.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A heating unit comprising a glass-like base of high dielectricstrength, good thermal shock resistance, chemical inertness, goodabrasion resistance and mechanical strength, and being transparent toinfrared radiation, a narrow continuous strip of multi-layer metallicfilm bonded to the base, the film being a composite material comprisingat least one inner layer of platinum having a relatively high resistancein the range of about 13 to 39 ohms per square, and an outer layer ofgold having a relatively low resistance in the range of about 0.5 and1.5 ohms per square, the platinum having a higher melting point than thegold to protect the gold so that the gold will not crystallize at hightemperatures of around 1200, the two film layers representingresistances that are arranged in parallel so that the total resistanceof the narrow strip is approximately equal to the low; resistance of theouter gold layer.

2. A resistance heating unit comprising a narrow continuous strip ofmulti-layer metallic film bonded to a dielectric base material, the filmhaving at least one inner layer of platinum having a relatively highresistance in the range of about 13 to 39 ohms per square, and an outerlayer of gold having a relatively low resistance in the range of about0.5 and 1.5 ohms per square, the platinum having a much higher meltingpoint than the gold to retard the crystallization and migration of thegold, the two film layers representing resistances that are arranged inparallel so that the total resistance of the narrow strip isapproximately equal to the low resistance of the outer gold layer.

3. A resistance heating unit comprising a narrow continuous strip ofmulti-layer metallic film bonded to a dielectric base material, the filmhaving at least one inner layer of platinum and an outer layer of gold,the gold being a relatively low resistance conductor of electricalcurrent while the platinum is a relatively high resistance material witha much higher melting point than the gold to retard the crystallizationand migration of the gold, the narrow strip of metallic film beingarranged in a serpentine pattern having electrical terminals at each endof the strip as well as one adjacent the mid-portion thereof.

4. A resistance heating unit as recited in claim 2 wherein thedielectric base material is a sheet of nearly pure silicon oxide that isetched on both sides, and the multilayer metallic film heater ispositioned on the underside of the base material.

5. A surface heating unit for an electric range comprising a dielectricplate member having a film heater in strip form bonded to the undersidethereof, one side edge of the plate having a down-turned tongue on whichthe terminal ends of the film heater are disposed, a reflector panpositioned under the plate to shield the terminals from the hightemperatures of the main portion of the film heater.

6. A surface heating unit as recited in claim 5 wherein the dielectricplate member is a glass with high resistivity and thermal shockresistance as well as being transparent to infrared radiation.

7. A surface heating unit as recited in claim 5 wherein the film heaterhas at least one layer each of platinum and gold where the platinum hasa much higher resistance and melting point than the gold to retard thecrystallization and migration of the gold.

References Cited in the file of this patent UNITED STATES PATENTS617,375 Voigt et al. Jan. 0, 1899 2,021,661 Kisfaludy Nov. 19, 19352,603,740 Buttero July 15, 1952 2,640,906 Haynes June 2, 1953 2,693,023Kerridge et al. Nov. 2, 1954 2,757,104 Howes July 31, 1956 FOREIGNPATENTS 486,639 Great Britain June 8, 1938 611,250 Great Britain Oct.27, 1948

